1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a system and method for allocating ranging slots in a BWA communication system using an Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) scheme.
2. Description of the Related Art
Research is being actively conducted on the 4th generation (4G) next generation communication system, to provide users with services guaranteeing various qualities-of-service (QoS) at a data rate of about 100 Mbps. Currently, the 3rd generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having poor channel conditions, and supports the data rate of a maximum of only about 2 Mbps even in an indoor channel environment having good channel conditions.
An Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system, one of the typical BWA communication systems, performs a ranging operation between a mobile station (MS) and a base station (BS) to make communication.
With reference to FIG. 1, a description will now be made of a system configuration considered in the IEEE 802.16 communication system known in the prior art.
FIG. 1 is a diagram schematically illustrating a configuration of a conventional IEEE 802.16 communication system.
Referring to FIG. 1, the IEEE 802.16 communication system has a multicell configuration including a cell 100 and a cell 150. The system includes a BS 110 for managing the cell 100, a BS 140 for managing the cell 150 and a plurality of MSs 111, 113, 130, 151 and 153. Signal exchanges between the BSs 110 and 140 and the MSs 111, 113, 130, 151 and 153 are achieved using an OFDM/OFDMA scheme.
A communication system using an OFDM scheme (hereinafter “OFDM communication system”) is similar in frame format to a communication system using an OFDMA scheme (hereinafter “OFDMA communication system”), but the OFDM communication system differs from the OFDMA communication system in that the full subcarrier interval in a particular symbol is used by one MS. One OFDMA frame is comprised of a plurality of OFDMA symbols, and each of the OFDMA symbols is comprised of a plurality of subchannels. Every OFDMA frame has a ranging channel, and each ranging channel is comprised of a plurality of ranging slots and one or more subchannels, and unique numbers of the subchannels constituting the ranging channel are included in an uplink MAP (UL-MAP) message.
The UL-MAP message, which represents uplink frame information, includes an Uplink Channel identifier (ID) field indicating an uplink channel ID used, an Uplink Channel Descript (UCD) count field indicating a count corresponding to a change in format of a UCD message including an uplink burst profile, and a Number of UL-MAP Element n field indicating the number of elements existing after the UCD count.
As a result, the OFDMA communication system requires a ranging process of determining a correct time offset between a BS and an MS and adjusting a power level. Rangings used in the IEEE 802.16 communication system are classified into initial ranging, maintenance ranging (or periodic ranging) and bandwidth request ranging.
1) Initial Ranging
The initial ranging, which is for synchronization acquisition for a BS and an MS, is performed to determine a correct time offset between the BS and the MS and adjust transmission power. That is, upon power-on, the MS performs the initial ranging in order to acquire synchronization with the BS by receiving such information broadcasted from the BS as a DL-MAP message, a UL-MAP message and a UCD message, and then to adjust the time offset and the transmission power with the BS.
2) Periodic Ranging
The periodic ranging refers to the ranging that is periodically performed by the MS to adjust channel conditions with the BS, after adjusting the time offset and transmission power with the BS through the initial ranging.
3) Bandwidth Request Ranging
The bandwidth request ranging refers to the ranging in which the MS requests bandwidth allocation to perform actual communication with the BS, after adjusting the time offset and transmission power with the BS through the periodic ranging.
With reference to FIG. 2, a description will now be made of a format of an uplink frame in a communication system using the OFDM and/or OFDMA scheme (hereinafter “OFDM/OFDMA communication system”).
FIG. 2 is a diagram schematically illustrating a format of an uplink frame in an OFDM/OFDMA BWA communication system, and in particular, a format of an uplink frame in an IEEE 802.16a/IEEE 802.16e OFDM communication system.
Referring to FIG. 2, an uplink frame 200 includes an initial ranging contention slot region 210 allocated for the initial ranging, a bandwidth request contention slot region 220 allocated for the bandwidth request ranging, and a plurality of uplink burst regions 230 and 240 including uplink data of MSs.
The initial ranging contention slot region 210 has a plurality of access burst intervals including actual initial ranging information, and a collision interval prepared for possible collision between the access burst intervals.
The bandwidth request contention slot region 220 has a plurality of bandwidth request intervals including actual bandwidth request ranging information, and a collision interval prepared for possible collision between the bandwidth request intervals.
The uplink burst regions 230 and 240 each include a plurality of burst regions so that uplink data can be transmitted for each of individual MSs, and each of the burst regions includes a preamble 231 and an uplink burst 233.
Each MS that desires to perform initial ranging randomly selects one slot from the initial ranging contention slot 210, and transmits an access request including information on the selected slot to a BS. However, if two or more MSs select the same initial ranging slot, collision occurs. In this case, the MSs retry the initial ranging after a delay of a predetermined back-off time in order to increase a ranging success rate at the next attempt. This back-off process is applied in the same way to the periodic ranging and the bandwidth request ranging as well as the initial ranging.
FIG. 3 is a signaling diagram schematically illustrating a communication process in a BWA communication system.
Referring to FIG. 3, upon power-on, an MS 320 monitors all frequency bands previously established thereto, and detects a pilot channel signal having the highest pilot carrier-to-interference and noise ratio (CINR). The MS 320 determines a BS 300 that transmitted the pilot channel signal having the highest pilot CINR, as its own BS where it is currently located, and acquires system synchronization with the BS 300 by receiving a preamble in a downlink frame transmitted by the BS 300.
If the system synchronization is acquired between the MS 320 and the BS 300, the BS 300 transmits a DL-MAP message and a UL-MAP message to the MS 320 in steps 301 and 303, respectively. The DL-MAP message is used to provide the MS 320 with information necessary for acquiring synchronization with the BS 300 by the MS 320 in a downlink and information on a format of a physical channel capable of receiving messages transmitted to MSs in the downlink using the information. The UL-MAP message is used to provide the MS 320 with such information as MS's scheduling period and a format of the physical channel in the downlink.
In the ranging process, the MS 320 transmits a Ranging Request (RNG-EQ) message to the BS 300 in step 305, and upon receiving the RNG-REQ message, the BS 300 transmits a Ranging Response (RNG-RSP) message including information for correcting frequency, time and transmission power, to the MS 320 in step 307.
FIG. 4 is a diagram schematically illustrating a back-off process at the collision of ranging slots in a conventional BWA communication system.
Although the back-off process described with reference to FIG. 4 can be applied to the initial, periodic and bandwidth request ranging processes, it will be assumed herein that the back-off process is applied to the initial ranging process, by way of example.
Referring to FIG. 4, one frame includes L ranging slots for initial ranging. In the case of a first frame, a third ranging slot among the L ranging slots was selected by 3 MSs to transmit RNG-REQ messages. The MSs transmitting the RNG-REQ messages at the third ranging slot will be assumed to be a first MS 401, a second MS 403 and a third MS 405.
Preferably, one ranging slot is occupied by one MS. However, when one ranging slot is selected by a plurality of MSs on a contention basis as shown in FIG. 4, the MSs cannot request the ranging at the corresponding ranging slot.
Therefore, only one of the first MS 401, the second MS 403 and the third MS 405 can perform initial ranging request at the third ranging slot, and the remaining MSs perform the random back-off process. For example, when the first MS 401 occupies the third ranging slot, the second MS 403 performs ranging request at a fourth ranging slot in a second frame by performing a random back-off process, and the third MS 405 performs ranging request at a second ranging slot in the second frame by performing a random back-off process.
Conventionally, a BS allocates ranging fields to MSs according to a fixed ranging slot field and a fixed ranging allocation period. That is, the BS must minimize the collision caused by slot sharing between MSs and an access delay caused by the back-off by maintaining a maximum number of slots as ranging attempt slots, since it cannot know how many MSs have previously performed ranging request. In other words, in the conventional ranging field allocation process, the BS previously determines a fixed number of ranging slots and a fixed ranging slot allocation period during system building or network design. However, excessive ranging attempts caused by an increase in number of MSs may lead to overload on the system, causing a ranging delay. In this case, the system operator should undesirably modify the fixed ranging slot field by redesigning the system. In addition, when the number of MSs decreases, the ranging slot field fixed during the initial system design causes a waste of radio resources.